Wireless transmissions are used for wireless networking among computing devices as well as in other settings, such as to support broadcast of radio and television content. To avoid interference between the signals of different users, different portions of the electromagnetic spectrum are allocated to different uses.
The radio spectrum has been divided administratively into two primary sectors: the licensed spectrum and the unlicensed spectrum. The licensed spectrum consists of frequencies that have been licensed to organizations, such as commercial broadcasters, for exclusive use by those organizations. For example, a portion of the spectrum may be licensed to a cellular communications company for use in communicating information representing voice and data calls placed by its subscribers, or licensed to a media broadcaster to transmit signals carrying audio and video data representing television content. The unlicensed spectrum, on the other hand, has been allocated for free use by the public. The unlicensed spectrum includes a band allocated for industrial, scientific and medical uses (the ISM band), which is frequently used for intra-computer communications. Though operation in these bands does not require a license, typically, use of the unlicensed spectrum requires operating in accordance with some regulations, such as maximum power output regulations, that are designed to minimize interference between users.
Despite this administrative distinction between licensed and unlicensed spectra, unused portions of the licensed spectrum are sometimes used for unlicensed communications. Unused portions may arise, for example, because, when frequency bands of the licensed spectrum are assigned to organizations, typically they are not assigned exactly consecutively. Instead, in a given location, there may be gaps between the licensed portions to prevent transmissions of two licensed users from interfering with one another. The unassigned band of the licensed spectrum is referred to as “white space.” These white spaces may be intended to provide frequency separation to reduce interference between the signals of licensed users. Additionally, white spaces may arise because, in a particular location, the licensed users do not collectively require the full spectrum.
White spaces of the licensed spectrum are sometimes used by unlicensed transmitters. By government regulation, use of white space in this way requires using techniques that prevent interference with signals of the licensed users. Generally, these techniques require an unlicensed wireless device to access a database of white space channels. The database contains information that indicates, for any location, which channels within a licensed spectrum are available for use as white space. Regulations prescribe the accuracy with which a wireless device must know its location in order to use information from the regulatory database to identify available white space channels.